overclocking the 555 timer

Updated 10 May 2016

I got 9.4Mhz clock out of the Micrel MIC1557 version of the ubiquitous 555. This was with 5V supply, no timing capacitor (relying on the internal capacitance of the device) and a 330 ohm timing resistor. This indicates that the internal delay of the device is about 55nS. Not bad, considering it is a low power design. The device is in a SOT23-5 surface mount package and needs a 0.1uF supply decoupling capacitor.

The table shows the frequencies I got with 10pF 1Kohm timing components The frequency at 5V was fairly stable from 4.6 to 6.8V.

Supply volts

Frequency MHz

2

2.7

3.3

6.58

5

7.5

The 555 was designed by Hans Camenzind Fascinating interview with him here:

The output of a 555 is a sawtooth but here is a circuit of a sine wave oscillator using the 555:

The inductor has properties of 64 ohms resistance. I initially developed it using the ecircuitcenter model of the 555 then built it and finally was gratified to see it work on the LTspice model of the 555. However the real frequency was 69kHz and this model give 79kHz. Below are the waveshapes you get at the junction of the threshold and trigger pins and the output. Unfortunately the output square wave is not symetrical. I got better results with the above circuit if I made R3 680 ohms as the output did not overshoot so far negatively. With a 15mH inductor (Farnell 233-3679) and 27nF capacitors it should do 10kHz. To get the frequency prediction more accurate in the model you could add the parallel capacitor given by the inductors self resonant frequency.